Selection of Surge Protective Devices

01 Definition and Classification of Surge Protective Devices

1.Definition of Surge Protective Device
A Surge Protective Device (SPD) is a non-linear protective device used in live electrical systems to limit transient overvoltages and divert surge currents. It is designed to protect electrical or electronic systems from damage caused by lightning, switching overvoltages, and inrush currents.

2.Classification of Surge Protective Devices
SPDs can be classified based on the characteristics of their non-linear components or according to their specific application requirements in different systems.

2.1 Classification Based on Non-linear Component Characteristics
(1) Voltage Switching Type SPD:
Exhibits high impedance under normal conditions and switches to low impedance upon the occurrence of a voltage surge. Common components include spark gaps, gas discharge tubes, silicon-controlled rectifiers (SCRs), or triacs. Also known as a "Crowbar Type" SPD, it features discontinuous voltage and current characteristics.

(2) Voltage Limiting Type SPD:
Exhibits high impedance under normal conditions, with impedance decreasing continuously as the surge current and voltage increase. Common components include metal oxide varistors (MOVs) and suppression diodes. Also known as a "Clamping Type" SPD, it exhibits continuous voltage and current characteristics.

(3) Combination Type SPD:
Composed of both voltage switching and voltage limiting components. Its characteristics may manifest as voltage switching type, voltage limiting type, or a combination of both, depending on the applied voltage.

2.2 Classification Based on Application Requirements in Different Systems
According to their purpose, SPDs can be categorized into: Power System SPDs, Signal System SPDs, and Antenna Feeder System SPDs.

 

02 Key Parameters for Selecting Surge Protective Devices

1.Selection of SPD Test Category
SPD test categories are classified into three types: Class I, Class II, and Class III. Generally, Class I or Class II SPDs are used in main distribution cabinets; Class II or Class III SPDs are suitable for sub-distribution panels and downstream applications. Specifications describe this as follows:

(1) For AC power supply lines entering a building, at the boundary between zones such as LPZ0A or LPZ0B and LPZ1 (e.g., the main distribution cabinet), a Class I or Class II tested SPD should be installed as the primary protection.
(2) At subsequent protection zone boundaries, such as distribution boards in branch circuits or power distribution cabinets in electronic equipment rooms, Class II or Class III tested SPDs may be installed as secondary protection.
(3) For particularly critical electronic information equipment power ports, Class II or Class III tested SPDs can be installed for fine protection. For information equipment using DC power supply, appropriate DC power line SPDs should be installed based on their operating voltage requirements.

2.Selection of Maximum Continuous Operating Voltage
(1)The Maximum Continuous Operating Voltage (Uc) is the maximum RMS voltage (or DC voltage) that can be continuously applied to an SPD. This value serves as a threshold for the SPD; when the operating voltage exceeds this value, the SPD activates, and leakage current increases exponentially.

(2)The selection of Uc depends on the system grounding type, system voltage, and power quality. The Uc value must be greater than the normal operating voltage. For example, in a TN system with 380/220V, the positive voltage deviation typically does not exceed 7%. However, in practice, China's voltage positive deviation often exceeds this value. Considering a 10% deviation and an additional 5% for aging factors, Uc should be ≥ 1.15Uo.

(3)In locations where the power supply system voltage deviation exceeds 10% or where harmonic effects cause an increase in the normal operating voltage amplitude, the Uc value of the SPD should be appropriately increased based on specific conditions. At the same time, the coordination between the overvoltage protection level (Up) and the protected equipment must be considered. Please refer to the figure below for details, where Uo represents the phase voltage and U represents the line voltage.

 

Minimum Values of Maximum Continuous Operating Voltage for Surge Protective Devices Depending on System Characteristics

SPD Connection	TN-S System	TN-C System	TT System	With Neutral Derived IT System	Without Neutral Derived IT System
L - N	≥ 1.15 Uo	N/A	≥ 1.15 Uo	≥ 1.15 Uo	N/A
L - PE	≥ 1.15 Uo	N/A	≥ 1.15 Uo	≥ √3 Uo	≥ U
L - PEN	N/A	≥ 1.15 Uo	N/A	N/A	N/A
N - PE	N/A	N/A	N/A	≥ 1.15 Uo	≥ 1.15 Uo
L - L	≥ 1.15 U	≥ 1.15 U	≥ 1.15 U	≥ 1.15 U	≥ 1.15 U

In DC systems, the Maximum Continuous Operating Voltage (Uc) of an SPD is approximately 1.5 times the rated voltage of the protected system (as an empirical value).

 

3. Selection of Impulse Current and Maximum Discharge Current

The impulse current (Iimp) is typically used as a parameter for the Class I classification action load test of an SPD, with its waveform usually being 10/350 μs.

The maximum discharge current (Imax) is used for the Class II classification test of an SPD. The specific selection depends on the exposure location and can be referenced in the table below.

Table 5.4.3-3 Recommended Values for Impulse Current and Nominal Discharge Current Parameters of Power Line Surge Protective Devices

Lightning Protection Level	Main Distribution Cabinet		Sub-distribution Cabinet	Equipment Room Distribution Cabinet and Ports Requiring Special Protection
	LPZ0 to LPZ1		LPZ1 to LPZ2	Subsequent Protection Zone Boundaries
	10/350μs (Class I Test)	8/20μs (Class II Test)	8/20μs (Class II Test)	8/20μs (Class II Test)	Composite Wave (1.2/50μs & 8/20μs) (Class III Test)
	Iimp (kA)	In (kA)	In (kA)	In (kA)	Uoc (kV) / Isc (kA)
A	≥ 20	≥ 80	≥ 40	≥ 5	≥ 10/≥ 5
B	≥ 15	≥ 60	≥ 30	≥ 5	≥ 10/≥ 5
C	≥ 12.5	≥ 50	≥ 20	≥ 3	≥ 6/≥ 3
D	≥ 12.5	≥ 50	≥ 10	≥ 3	≥ 6/≥ 3

 

4,Selection of Voltage Protection Level

The Voltage Protection Level (Up) is a performance parameter of an SPD that limits the voltage between its terminals. For voltage-switching type SPDs, it refers to the maximum spark-over voltage under a specified rate of rise; for voltage-limiting type SPDs, it refers to the maximum residual voltage under a specified current waveform.

The sum of the Voltage Protection Level (Up) of the SPD and the induced voltage across its connecting leads must be less than the rated impulse withstand voltage (Uw) of the system and equipment insulation (refer to the figure below for Uw values) and should preferably not exceed 80% of the withstand voltage level of the protected equipment.

Table 5.4.3-1 Rated Impulse Withstand Voltage (Uw) Values for Various Equipment in a 220V/380V Three-Phase Distribution System

Equipment Location	Power Supply Incoming Terminal	Branch Distribution Circuit Equipment	Utilization Equipment	Electronic Information Equipment Requiring Protection
Impulse Withstand Voltage Category	IV	III	II	I
Uw (kV)	6	4	2.5	1.5

 

5. Overall Principles for Surge Protective Device (SPD) Selection Design

(1) Holistic Principle.
Consider the lightning protection for the entire electromechanical system of the security engineering project as a unified protective object. It is essential to address the lightning protection of each subsystem while ensuring effective coordination among the various low-voltage subsystems and between them and the power supply and distribution subsystems. The system configuration should be systematic, economically reasonable, safe, reliable, and include appropriate redundancy.

(2) Integrated Prevention.
Fully utilize traditional lightning protection technical measures such as "potential equalization, shielding, air termination, diversion, grounding, and protection." Select reliable air termination devices and Surge Protective Devices (SPDs) to implement a comprehensive and dependable technical solution.

(3) Rational Selection.
Based on the specific geographical environment characteristics of the security engineering project and the lightning protection zone in which the electromechanical equipment is located, select appropriate SPD electrical performance parameters and product functionalities.

 

03 Selection Parameters for Each Level of Surge Protective Devices

1. SPD1

If the first-level surge protective device SPD1 is installed in the main power distribution room of a building, three voltage-switching type SPDs are generally sufficient to meet the system requirements. Even under China's current power supply system, which adopts a TN-S configuration, the Neutral (N) and Protective Earth (PE) are bonded at a single point in the main distribution room. Therefore, installing three voltage-switching type SPDs in the main distribution cabinet of the power distribution room is effective. It is not necessary to install an additional SPD between N and PE.

Recommended selection parameters for SPD1 are as follows:

Maximum Continuous Operating Voltage: Uc = 440V

Maximum Discharge Current: Generally designed and calculated according to code requirements or referenced from standards.

Protection Level: Up ≤ 2.5kV

Response Time: tA ≤ 100ns

2. SPD2

The second-level power surge protective device is the most widely used product. Situations where lightning electromagnetic pulses can induce surge currents exceeding 20kA on power lines intruding into LPZ1 zones are relatively rare. Therefore, the nominal discharge current (In) for the second-level SPD, as a voltage-limiting type SPD, is typically specified between 20~40kA.

Recommended selection parameters for SPD2 are as follows:

Maximum Continuous Operating Voltage: Uc = 260~320V

Nominal Discharge Current (8/20μs): In = 20kA

Maximum Discharge Current (8/20μs): Imax = 40kA

Protection Level: Up ≤ 1.5kV

Response Time: tA ≤ 25ns

3. SPD3

The third-level power surge protective device is typically installed at the front end of important equipment, serving as so-called fine protection.

Recommended selection parameters for SPD3 are as follows:

Maximum Continuous Operating Voltage: Uc = 255V

Nominal Discharge Current (8/20μs):

In (L→N) = 3kA

In (N→PE) = 5kA

Voltage Protection Level (Up):

L→N ≤ 1.25kV

L→PE ≤ 1.5kV

Response Time: tA ≤ 25ns

04 Installation Requirements for Surge Protective Devices

The three fundamental requirements for the installation and use of SPDs:

(1) After the installation of an SPD, under normal operating conditions without surges, the SPD should not adversely affect the operation of the electrical (or electronic) system.

(2) After the installation of an SPD, in the event of a surge, the SPD should be capable of withstanding the expected lightning current without damage, while also clamping the surge voltage and diverting the surge current.

(3) After the surge current has passed, the SPD should promptly return to a high-impedance state and interrupt the power-frequency follow current.

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